# Non-linear model for SPICE

W

#### wombat

Jan 1, 1970
0
Hi All,

I need to create a resistor whose resistance varies according to an
equation with the variables of the top and bottom voltage (ie. the
voltage across it). I use Altium (was Protel) but I suspect the
fundamental process is the same. My main concern is that it is what
excel would refer to as a circular reference but I don't understand the
inner workings of SPICE tools to know whether it's feasible.

Cheers,
wombat

J

#### Jim Thompson

Jan 1, 1970
0
Hi All,

I need to create a resistor whose resistance varies according to an
equation with the variables of the top and bottom voltage (ie. the
voltage across it). I use Altium (was Protel) but I suspect the
fundamental process is the same. My main concern is that it is what
excel would refer to as a circular reference but I don't understand the
inner workings of SPICE tools to know whether it's feasible.

Cheers,
wombat

In PSpice this is done with behavioral models, for example "GVALUE",
which is a current source written as an equation relating to the end
voltages.

In other Spice's I believe these are called "B" sources.

...Jim Thompson

K

#### Kevin Aylward

Jan 1, 1970
0
wombat said:
Hi All,

I need to create a resistor whose resistance varies according to an
equation with the variables of the top and bottom voltage (ie. the
voltage across it). I use Altium (was Protel) but I suspect the
fundamental process is the same. My main concern is that it is what
excel would refer to as a circular reference but I don't understand
the inner workings of SPICE tools to know whether it's feasible.

In generic spice you use the "B Source".

e.g

b1 1 0 i=2.4*V(1,0) + 3.5*V(1,0)*V(1,0)*

This makes the current through nodes 1,0 be a function of the voltage
across it, i.e. a non-linear resister.

You should make a .subckt model with all the details in it etc...read
manual etc..

Kevin Aylward B.Sc.
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

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